An accurate knowledge of the composition of coal or coke is very important in many aspects of production or beneficiation and in the utilisation of coal or coke in order to ensure a uniform product and/or batch.
Coal and coke consists of coal matter (oxygen and combustible materials, carbon, hydrogen and a little nitrogen and sulphur) and mineral matter (mainly of incombustible aluminium and other silicates, and a little iron sulphide which is partly combustible). Coal ash is the oxidised incombustible residue from the combustion of coal, and is closely correlated with the content of mineral matter.
An accurate knowledge of the mineral content of coal is very important in many aspects of coal production, preparation and utilisation. It is especially advantageous to have a continuous monitor of mineral content of coal during coal washing and blending operations, production of coke, and monitoring feed in installations for power generation, metallurgical smelting and gas production.
Coal as mined has a variable heterogeneous mineralogy and usually a wide particle size distribution. The coal is washed to reduce mineral content and to ensure a more uniform product, and blended to obtain specific characteristics suitable for a particular application requirement. When the mineral content can be monitored continuously, washing and blending can be controlled better to ensure a more uniform and lower mineral content and therefore more appropriate characteristics.
In the specification and claims when describing methods of determining ash content of coal a reference to coal is also a reference to coke. Also, since the content of mineral matter is closely related to the content of ash, the content of one can be determined at least approximately from a measurement of the other.
It is known to determine the ash content of coal gravimetrically by burning a known amount of coal and weighing the residue. In order to reduce errors, a large sample is taken and ground and the sample size reduced in accordance with standard sampling procedures. This method does not permit a rapid continuous monitor of ash content.
Continuous and rapid methods for determining the ash content of coal are known and depend on scatter of .beta. particles, or transmission or scatter of X- or .gamma.-rays. Such methods are described in Cameron J. F., "Measurement of ash content and calorific value of coal with radioisotope instruments" O.R.N.L. 11C-10 Vol. 2 P 903, Cameron J. F., Clayton C. G., "Radioisotope Instruments Volume 1" International Series of Monographs in nuclear energy Volume 107, Pergamon Press (1971), Kato M., "Present status of research and application of low-energy X-and gamma-ray sources in Japan" O.R.N.L. 11C-10 Vol. 2 P 723, Rhodes J. R., "Ore and Coal Analyses using radioisotope techniques" O.R.N.L. 11C-5, P 206 and Vasilev A. G. et al, "Express ash analyser based on the recording of forward scattered gamma rays" Koksi Khimiya 1974 No. 5, P 52. The basis of these methods is that the mean atomic number of the mineral matter constituents is higher than that of the coal matter, and that .beta. and .gamma.-ray interactions with atoms are atomic number dependent. The mean atomic number of mineral matter, however, is not constant, and in practice variations in iron content of the mineral matter cause considerable errors in determination of ash using the above methods.
Neutron techniques can, in principle, be used to determine ash content of coal because these techniques can be used to determine concentrations of the more abundant elements of the coal. Such methods are described in Cameron J. F., "Measurement of ash content and calorific value of coal with radioisotope instruments" O.R.N.L. 11C-10 Vol. 2 P 903. Neutrons, and the .gamma.-rays produced by neutron interactions with the coal, both penetrate large volumes of coal and hence neutron techniques can be used with relatively large particles of coal. However, determination of concentrations of all the elements required for accurate determination of ash content would, in practice, be very complex because some of the neutron techniques for individual elements are complex. Techniques are, in practice, relatively simple for only a limited number of elements, e.g., iron by neutron capture .gamma.-ray techniques as described by FMC Corporation, "Analysis of coal with Cf-252", pages 37 to 39 in Californium Progress, Jan. 20, 1976 and by Ljunggren K. and Christell R., "On-line determination of the iron content of ores, ore products and wastes by means of neutron capture gamma radiation measurement", page 181 in Nuclear Techniques in Geochemistry and Geophysics, IAEA, Vienna 1976.
In methods based on scatter of .beta. particles, the intensity of particles scattered from a material is related to the mean atomic number of the material. As the mean atomic number of coal increases with mineral matter content the intensity of .beta. particles scattered from coal is proportional to the ash content. However, large errors occur in this method through variations in iron and moisture content.
In methods based on transmission of X-rays or low energy .gamma.-rays the intensity of the radiation transmitted through a sample of fixed weight per unit area decreases with increasing mass attenuation coefficient of the bulk material. At energies less than about 100 keV the mass attenuation coefficient changes rapidly with atomic number, which means that the transmitted intensity is sensitive to the coal composition.
The ratio of the intensity (I) of a collimated beam of radiation transmitted through a coal sample of thickness (x) and bulk density (.rho.) to the intensity (I.sub.o) in the absence of coal is EQU I/I.sub.0 = exp (-(.SIGMA..mu..sub.i .multidot.C.sub.i) .rho..multidot.x) (1)
where .mu..sub.i and C.sub.i are the mass absorption coefficient and concentration (weight fraction) of the i.sup.th element in the coal respectively. Now EQU .SIGMA..mu..sub.i .multidot.C.sub.i = .mu..sub.coal matter .multidot.C.sub.coal matter EQU + .mu..sub.mineral matter .multidot.C.sub.mineral matter ( 2)
where C is concentration and EQU C.sub.coal matter + C.sub.mineral matter = 1 (3)
Hence if coal samples to be analysed have mineral matter of essentially constant composition and if the weight per unit area (.rho..multidot.x) of coal is separately measured, and results combined with equations (1), (2) and (3), the concentration of mineral matter and hence the closely correlated ash content are determined.
A high sensitivity to variations in mineral matter content can be obtained with methods employing transmission because the sensitivity to content of mineral matter is proportional to (x) in equation (1).
In methods based on scatter of X- or .gamma.-rays, the intensity (I) of radiation scattered from the coal depends on the probability of coherent and Compton scattering, and absorption of X- or .gamma.-rays, within the sample. The optimum energy range for maximum sensitivity to ash is 10 to 20 keV, and in this case EQU I .apprxeq. k/.SIGMA.(.mu..sub.i .multidot.C.sub.i) (4)
wherein .mu..sub.i and C.sub.i are the mass absorption coefficient and concentration respectively for the i.sup.th element in the coal sample, and k depends on overall geometry and detection efficiency, and output of the .gamma.- or X-ray source. If coal samples to be analysed have mineral matter of essentially constant chemical composition, then mineral matter content and the closely correlated ash content are determined.
Variations in iron content of the mineral matter affect both the X- and .gamma.-ray transmission and scatter methods as follows:
(1) If the X-ray energies are chosen below the iron K shell absorption edge (7,1 keV), the mass absorption coefficient of iron and the mean for the other constituents of mineral matter is about the same and the mineral matter content and hence ash is determined with reasonable accuracy. However, these low energy X-rays are strongly absorbed and the coal must be finely ground (less than 0,3 mm) so that measurements can be made. This is a severe limitation in practice for on-line determination.
(2) If the X-ray energies are chosen above the iron K shell absorption edge, compensation must be made for iron content because iron absorbs far more per unit weight than the absorption per unit weight of the other constituents of the mineral matter. The only compensation currently used for iron depends on excitation of iron K X-rays. This method can only be used on finely ground coal since the K X-rays are greatly absorbed in less than 1 mm thickness of coal.
Hence, unless the very unusual case occurs of essentially no variation of iron content of the mineral matter, the content of mineral matter and hence ash can only be determined accurately by currently used .gamma.-ray and X-ray techniques if the particle size of coal is very small.
It would be expected therefore from the prior art that accurate determination of ash content of coal by X-ray and low energy .gamma.-ray techniques would be unlikely unless the iron content of the ash was constant or the coal particles were finely ground. It would also be expected from prior art that the use of neutron techniques to determine ash content accurately would not be a practical proposition particularly for on-line systems.